In general, there is a demand for efficient signal processing for audio applications. For example, the persistent trend toward miniaturization and portability of devices equipped with loudspeakers is raising the demands on signal processing algorithms. Consumers expect audio performance to improve or at least not drop even as playback devices decrease in size, reducing their acoustical performance.
In the field of audio-signal processing for consumer audio devices, it is very common to employ several kinds of dynamic-range processing to adapt the audio signal to the available hardware and the intended playback environment. In general, when the playback devices are miniature loudspeakers such as those used in mobile phones, it is desirable to apply dynamic-range compression in order to boost low level parts of the audio signal in order to boost audibility and loudness. Furthermore, it is desirable to employ limiting for preventing the audio signal from exceeding the maximum allowed levels of digital amplitude and loudspeaker membrane excursion.
One of the most important types of processing for miniature loudspeakers is limiters. Limiters prevent signals from exceeding a threshold amplitude-level, they work by applying an attenuation gain commensurate with the amount by which a given signal exceeds the threshold level. The time for the limiter to reach full attenuation is often called attack time and is typically used in combination with a look-ahead time of comparable size. The time for the limiter gain to return to neutral gain after a moment of suppression is often called release time.
Traditional limiters suffer from several types of artifacts which can make the audio sound bad in different ways.
Setting limiter time constants normally involves a compromise between a loud but distorted output on one hand, and a less distorted but weaker output on the other hand. In general, longer time constants produce a more natural sounding result, but at the expense of loudness since the gain remains low for longer times. At one extreme, setting the attack and release times to zero results in clipping of the signal to the threshold level. The sound will be very loud but also very distorted due to the strong high-frequency content in the gain signal which modulates the input as it is applied. At the other extreme, a very long release time reduces the amount of high-frequency content in the gain signal while at the same time reducing loudness too much for many applications. Another problem with long release times is that it can cause unnecessary over-suppression over a period of time after the suppression of a transient part of the audio signal. This artifact is sometimes called “hole punching”. Moderate release time settings also have their problems; namely audible modulation in the form of so called “pumping”, an audible change of level of the whole signal as the gain quickly recovers after suppression. Pumping is especially audible on signals with transient high-level sounds embedded in wide-band background sounds. For low-frequency periodic signals which overshoot the threshold level, limiters with short release times can trigger on each cycle of the waveform and cause a distorted sound as they modulate the periodic signal. A general artifact of traditional time-domain limiters is that they suppress the whole frequency range of the signal, which can result in a dull or muffled sound.
There is thus a general need for solutions that may eliminate or at least reduce some or all of the described processing artifacts.